dragonworks
Banned
around here? HELP:
who is the amp guru
- Thread starter dragonworks
- Start date
Steenamaroo
...
I'm not sure there is one.......
sup?
sup?
jimmys69
MOODerator
Someone who makes a bunch of money, recording great players, in a world, far far away.....
His name is.........
wait for it..........
Hell, I don't know.
PRHunt
I void warranties
Might help to spell out exactly what help you need???
dragonworks
Banned
I am trying to resurrect an old preamp and have very limited knowledge and unable to obtain a schematic.
PRHunt
I void warranties
I am NOT the guru, but do you have access to any test equipment - multi-meter or volt ohm meter for example?
Also, what is the preamp? And what is the problem - no power, no signal, distortion???
Hopefully someone like ecc83 will wander by and pick this up...
But the more descriptive information you provide, the better.
Also, what is the preamp? And what is the problem - no power, no signal, distortion???
Hopefully someone like ecc83 will wander by and pick this up...
But the more descriptive information you provide, the better.
Steenamaroo
...
Yeah man, what's the preamp?
dragonworks
Banned
I hope this helps.
It is a teisco stereo preamp. Two channel, one clean, one with tremolo and reverb, both with bass, mid and high. I have a clean signal coming through both channels. No reverb or tremolo on channel two though. I have meters, scope, AF generator. I can see the signal on the plate and grid for the input to the reverb tank, I can see the signal at the input of the tank, I see a very weak and distorted signal coming out of the tank, it may not be a signal at all, but just eddy currents or gremlins maybe, I am new to all of this. I can see no signal at all at the output of the reverb tube, either on the plate or the grid. All plates etc. appear to have good DC voltage on them. There is zero DC on the grids I think. The input side of the rever tank also goes to a footswitch, the footswitch I think routes the clean signal back to the output? The output from the tank goes to the deapth control for the reverb, which then gets passed through some resistors and caps that are tied in somehow through the volume and tone controls? It goes to a rail and I am not sure how it gets passed to the out put grid of the tube? I am not sure the tank is any good. It is hard wired, three section it appears, the input comes into the right section and jumped to the other two sections then going to the footswitch. The output section on the tank has three outputs tied together that then goes to the depth control. I have tried the verb with the footswitch on and off. All three inputs and all three outputs also have individual grounds.
As for the tremolo circuit, I can see a signal, a strong one, but I don't know what it is supposed to look like on the scope? When I turn the depth pot I can see a change but not really with the speed control. Both plates for the tube are hooked up to one side of a small transformer. The other side of the transformer has two resistors wired to it which then goes to the output for channel two. I am not sure how the tremolo circuit is supposed to work yet.
There are five tubes total, it was given to me with five 12ax7s in it. Four out of the five tubes are using both sections, one tube is only using half the tube. I am pretty sure I have identified the four tubes that use both sections as channel one and two preamp, one tube for trem, one for verb.
That leaves the tube using only one half that I am not sure of what its purpose is? I can see a change in amplitude on the trem tube and the unidentified tube when turning the volume knob on channel two. I have a bunch of 12AX7s and have tried multiple tubes.
free dogman.
It is a teisco stereo preamp. Two channel, one clean, one with tremolo and reverb, both with bass, mid and high. I have a clean signal coming through both channels. No reverb or tremolo on channel two though. I have meters, scope, AF generator. I can see the signal on the plate and grid for the input to the reverb tank, I can see the signal at the input of the tank, I see a very weak and distorted signal coming out of the tank, it may not be a signal at all, but just eddy currents or gremlins maybe, I am new to all of this. I can see no signal at all at the output of the reverb tube, either on the plate or the grid. All plates etc. appear to have good DC voltage on them. There is zero DC on the grids I think. The input side of the rever tank also goes to a footswitch, the footswitch I think routes the clean signal back to the output? The output from the tank goes to the deapth control for the reverb, which then gets passed through some resistors and caps that are tied in somehow through the volume and tone controls? It goes to a rail and I am not sure how it gets passed to the out put grid of the tube? I am not sure the tank is any good. It is hard wired, three section it appears, the input comes into the right section and jumped to the other two sections then going to the footswitch. The output section on the tank has three outputs tied together that then goes to the depth control. I have tried the verb with the footswitch on and off. All three inputs and all three outputs also have individual grounds.
As for the tremolo circuit, I can see a signal, a strong one, but I don't know what it is supposed to look like on the scope? When I turn the depth pot I can see a change but not really with the speed control. Both plates for the tube are hooked up to one side of a small transformer. The other side of the transformer has two resistors wired to it which then goes to the output for channel two. I am not sure how the tremolo circuit is supposed to work yet.
There are five tubes total, it was given to me with five 12ax7s in it. Four out of the five tubes are using both sections, one tube is only using half the tube. I am pretty sure I have identified the four tubes that use both sections as channel one and two preamp, one tube for trem, one for verb.
That leaves the tube using only one half that I am not sure of what its purpose is? I can see a change in amplitude on the trem tube and the unidentified tube when turning the volume knob on channel two. I have a bunch of 12AX7s and have tried multiple tubes.
free dogman.
Last edited:
ranjam
New member
Meaning you can hook up the preamp to a power amp and get output from either 'channel'? With the tone and volume controls working? So far, so good.
Meaning the controls do nothing, and no effect is heard?
Well, you have the right tools. Now to trouble shoot. Usually, but not always, the reverb is coupled to the pan via a transformer. Rarely is it a capacitor, but you never know. The pan should have the two RCA cables connected. Remove the pan, and meter the input and output to the pan itself. You have to go inside the pan, and measure right on the RCA jacks. You should get real low readings; maybe 10Ω on the input and 47Ω on the output. If this is so, the pan is good. Now leave the pan out, and put a signal into the preamp. Scope the RCA cables coming from the preamp. One will be the output of the reverb (from the transformer), but it goes to the 'input' of the tank (if that makes sense, great). Signal? Great. Now attach your generator to the 'other' RCA cable. That's the input to the reverb recovery stage, coming from the 'output' of the tank. Do you get a signal? If so, great. If not, you have a recovery stage problem. Could be a tube, could be an open Plate load resistor, capacitor, etc. Divide and conquer.
The tremolo is just a low frequency oscillator; usually about 10Hz or so. Look right around the speed control; it goes to the tube, through a few capacitors. Now you will see three capacitors and a couple of resistors bunched together around that tube. Maybe the capacitors are .02uF or .01uF, and the resistors are 220K, 470K, or around there. If you have an analog meter, great. Put it on the Plate of the oscillator tube. The needle should swing back and forth, varying with the 'speed' setting. If not, just 'shotgun' it, and change all three capacitors.
That's about all I got. Without a schematic, it is tricky. But you can do it. Go slow, and divide and conquer.
dragonworks
Banned
As I stated, the reverb tank is hard wired, no rca plugs. The signal to the input of the tank, as far as I can tell, does not go through a transformer, but through a resistor cap network to get there. On the other side of the tank I have either no signal or a very weak one.
jimmys69
MOODerator
dragonworks
Banned
I beg to differ.You do know that dogman is free right? Well, mostly.
Can I unsolder the in and out on the reverb tank then run it through my Super Reverbs tank? The reverb tank is enclosed, it cannot be opened I don't believe. It is about six inches long and three to four inches wide and only about maybe and inch and quater to an inch and a half high. There should be a picture attached. The reverb tank is the large metal box on the right with the old orange paper around it.
free dogman
Attachments
Steenamaroo
...
I'm a relative noob here, but surely you can bypass it for testing, right? Or remove it from the circuit and test it on its own?
jimmys69
MOODerator
ranjam
New member
That is the weirdest reverb tank I have ever seen. If you can trace it, knowing which are the input leads and which are output leads would be a huge help. I would ask around and find the oldest tube service guy in your town. Maybe he knew these. Who sold Teisco in your town back in the day? Someone knows this unit. But for me, I'd spend a lot of time messing with the wiring trying to figure input/output.
A rough idea of how to do an RC coupled reverb is here; Channel Road Amplification: Transformerless Reverb Driver
I guess look for the signal at the plate, to the coupling capacitor, and into the pan. From the pan, follow a lead and try and connect a signal to wherever the pan leads you to. Sometimes I'll even just touch the lead with a finger as I turn the reverb volume control. You then know the recovery circuit works. But you have to be sure there is no B+ here!
A rough idea of how to do an RC coupled reverb is here; Channel Road Amplification: Transformerless Reverb Driver
I guess look for the signal at the plate, to the coupling capacitor, and into the pan. From the pan, follow a lead and try and connect a signal to wherever the pan leads you to. Sometimes I'll even just touch the lead with a finger as I turn the reverb volume control. You then know the recovery circuit works. But you have to be sure there is no B+ here!
dragonworks
Banned
It has input and output labeled right on it. I think I am going to clip the leads and remove it and test it and see if can come apart. There isn't a tube guy within 50 miles of here that isn't backed up for six months.
Steenamaroo
...
But waiting til you say something really cool.
Everything I say is really cool.
Where have you been?
Plus Arcadeko maxed me out.
I've decided max rep is the point where you cease to be polite.
Steenamaroo
...
Hey man,
I'm only asking incase this helps others who might diagnose.
You say it's a stereo preamp and you have reverb on one channel, but not the other.
Then you say you see a strong signal at the reverb tank input, but not at the output.
Are you talking about the defective channel only, or in general?
Maybe this is a stupid question, but is the reverb tank going to have two separate paths for true stereo operation, or is it a summed kind of thing?
The reason I ask is this.
Presumably it has to be a stereo effect otherwise your l+r outputs would be summed.
If it is, you could swap over the channel 1+2 outputs to see what changes. If the fault switches channel there's a problem with the tank.
If nothing, swap back then swap the channel 1+2 inputs. If the fault now switches channel, it exists before the tank.
That way you could rule out the tank without removing or opening it.
Beyond that, because it's stereo and one channel works, you should be able to trace the l+r paths point by point until you find a discrepancy. Maybe a dud cap or whatever.
Perhaps you've been through this.
Apologies if I'm talking crap here.
EDIT: oh, and if you haven't already, confirm that all pots and switches are working. Give them all a good clean to be sure.
If I had a £1 for every time a switch went failed........
I'm only asking incase this helps others who might diagnose.
You say it's a stereo preamp and you have reverb on one channel, but not the other.
Then you say you see a strong signal at the reverb tank input, but not at the output.
Are you talking about the defective channel only, or in general?
Maybe this is a stupid question, but is the reverb tank going to have two separate paths for true stereo operation, or is it a summed kind of thing?
The reason I ask is this.
Presumably it has to be a stereo effect otherwise your l+r outputs would be summed.
If it is, you could swap over the channel 1+2 outputs to see what changes. If the fault switches channel there's a problem with the tank.
If nothing, swap back then swap the channel 1+2 inputs. If the fault now switches channel, it exists before the tank.
That way you could rule out the tank without removing or opening it.
Beyond that, because it's stereo and one channel works, you should be able to trace the l+r paths point by point until you find a discrepancy. Maybe a dud cap or whatever.
Perhaps you've been through this.
Apologies if I'm talking crap here.
EDIT: oh, and if you haven't already, confirm that all pots and switches are working. Give them all a good clean to be sure.
If I had a £1 for every time a switch went failed........
dragonworks
Banned
it is stereo. It has a separate output for each channel. You have a switch for stereo or mono which works. There is only verb and trem on channel two. I have checked the pots with a meter and they are working. I am going to take the reverb tank out, I think it is bad , and test it. I was going to buy a small reverb tank anyways for future testing. Included is a picture of the front of the amp and it can be seen by the decal line above the controls for channel one, from input one to the bright switch for input one, that channel one does not include the rev. and trem. Above channel two you can see a line connecting the input all the way across showing that channel two has rev and trem. I have also confirmed this through testing and tracing the wiring. The quarter I stuck on top is for size reference.
Free dogman
Free dogman
